The invention relates to steam turbine control systems, more particularly to a control system for an extraction type steam turbine.
A common aspect of many industrial environments is the required simultaneous provision of adequate process steam and electric power. Extraction turbines allow a portion of their inlet steam flow to be directed to a process steam header by use of an extraction valve. They are widely used in industrial environments for cogeneration of process steam and electric power requirements because of their ability to accurately match these requirements in a balanced and stable fashion. In any given industrial plant, these requirements vary over time and an extraction turbine control system attempting to provide and match these requirements must respond accordingly.
Industrial utilization of extraction turbines requires appropriate adjustment of front-end extraction turbine control valves and the extraction valve. These adjustments are made through application of well-known valve position control loop technology.
A control loop is established by a combination of signals, including one representing the desired level of turbine operation, and one representing the existing level of turbine operation. A prior art analog controller functions in the control loop to compare these two signals, and noting any discrepancy, it operates to automatically bring the turbine operation to that level required to balance these signals. The particular combination of signal elements in a control loop reflects the control strategy used by the system designer. The combined operation of several control loops achieves the overall control philosophy used in the control system design.
The majority of extraction turbines in service are used in the industrial area--steel mills, refineries, paper mills, sewage treatment plants, etc., where in the past, generation of electricity by the extraction turbine was a byproduct and not really a necessity. The major use of the extraction turbine in these cases was for process steam availability. The extraction process steam is used to feed heaters in the plant, such as auxiliary heaters, furnace heaters and building heaters. It is used to power steam-driven pumps and is also used in various quenching processes associated with steel mill operations, such as coke-quenching and quenching of hot metal strip as it exits the rolling mill.
Prior art extraction turbine control systems have emphasized process steam extraction control at the expense of electric power output or megawatt control, that is, they have achieved tight extraction control while allowing megawatt output to deviate and float to a level consistent with a given process steam extraction requirement. Often, a complex, lengthy and delicate valve readjustment procedure was performed by an operator in a local control mode to bring megawatt output back to a desired level after having deviated due to a previous adjustment in the process steam extraction level via the extraction valve controller. A major difficulty of this readjustment procedure was presented by the requirement that it was performed so as to avoid a process upset, that is, that it was bumpless.
The operator's readjustment procedure was further complicated by the need to readjust settings due to the drift introduced by prior art analog control system circuitry which depended on discrete electronic components such as operational amplifiers, capacitors, diodes and resistors, etc. These circuits were prone to drift out of calibration over time and with temperature variations.
With unceasing increases in the costs of energy, personnel and equipment, the inadequacies of older extraction turbine control strategies have become magnified. The potential for operating cost reductions may be available through the application of industrial energy management systems. These optimization systems are arranged to provide the front-end plant boiler controls with the steam pressure, steam flow, and electrical energy requirements for the entire industrial plant. In order for plant optimization to occur, the boiler controls must be able to transmit to the extraction turbine control system the required level of extraction steam pressure and/or flow and/or megawatt output. Use of the boiler control system as a remote control system to automatically send into the extraction turbine control system all of the various process setpoints requires the provision of an extraction turbine control system capable of responding to them and moving its operational level in a bumpless fashion, without the need for operator intervention. The extraction turbine becomes a more important factor in this case especially in the cogeneration sense where power is being sold and delivered to the utility power grid. Now, tighter control of megawatt output becomes a more important function than it has been in the past.
It can be seen that prior art extraction turbine control systems reflected control strategies which did not fully exploit the extraction turbine capabilities noted earlier. It would therefore be desirable to provide a method for selection, from multiple available control loops, a particular control loop or combination of control loops reflecting a particular control strategy or strategies. It would also be desirable to provide a simplified method of extraction turbine control to fully utilize the capabilities of the extraction turbine in meeting process steam and electrical energy requirements. It would also be desirable to provide an extraction turbine control system that makes more efficient use of the extraction turbine by achieving tight control of extraction steam requirements and tight control of megawatt output through megawatt output correction during a process steam extraction mode. It would also be desirable to provide an extraction turbine control system with control loops that are free from drift in calibration of circuit components, thereby reducing periodic maintenance requirements. It would also be desirable to provide an extraction turbine control system that is capable of accepting remotely generated optimization setpoint signals and adjusting its operational level in accordance therewith, without the need for operator intervention once the operator has chosen a remote mode. Such a control system would enable the realization of front-end boiler fuel cost reductions because of the smoother boiler operation associated with better and more stable extraction turbine control.